Magnetic resonance imaging was introduced in the 1980s, and has developed into a major global imaging modality.
Clinical MRI depends for its success on the generation of strong and pure magnetic fields. A major specification of the static field in MRI is that it has to be substantially homogeneous over a predetermined region, known in the art as the “diameter spherical imaging volume” or “dsv”. Errors less than 20 parts per million peak-to-peak (or 10 parts per million rms) are typically required for the dsv.
MRI equipment has undergone a number of refinements since the introduction of the first closed cylindrical systems. In particular, improvements have occurred in quality/resolution of images through improved signal to noise ratios and introduction of high and ultra high field magnets. Improved resolution of images, in turn, has led to MRI being a preferred choice for for both structural anatomical and functional human MRI imaging.
The basic components of a typical magnetic resonance system for producing diagnostic images for human studies include a main magnet (usually a superconducting magnet which produces the substantially homogeneous magnetic field (the B0 field) in the dsv), one or more sets of shim coils, a set of gradient coils, and one or more RF coils. Discussions of MRI, can be found in, for example, Haacke et al., Magnetic Resonance Imaging: Physical Principles and Sequence Design, John Wiley & Sons, Inc., New York, 1999. See also Crozier et al U.S. Pat. Nos. 5,818,319, 6,140,900 and 6,700,468, Dorri et al U.S. Pat. Nos. 5,396,207 and 5,416,415, Knuttel et al U.S. Pat. No. 5,646,532, and Laskaris et al U.S. Pat. No. 5,801,609, the disclosures of which are incorporated herein in their entireties.
Whole body MRI magnets are typically around 1.6-2.0 meters in length with apertures in the range of 0.6-0.8 meters. Normally the magnet is symmetric such that the midpoint of the dsv is located at the geometric center of the magnet's structure along its main axis. Not surprisingly, many people suffer from claustrophobia when placed in such a space. Moreover, the large distance between the portion of the subject's body which is being imaged and the end of the magnet system means that physicians cannot easily assist or personally monitor a subject during an MRI procedure.
In addition to its effects on the subject, the size of the magnet is a primary factor in determining the cost of an MRI machine, as well as the costs involved in the siting of such a machine. Another important consideration is the volume of helium needed to maintain the system at cryogenic temperatures. Due to their large size, such whole body magnets are expensive for use in producing images of small sizes of objects, such as, heads, extremities and neonates, etc.
Known superconductive head magnets includes those disclosed in U.S. Pat. Nos. 5,396,207 and 5,416,415 issued to Dorri et al, as well as U.S. Pat. No. 5,801,609 issued to Laskaris et al. Those magnets have limited applications and are mostly suitable for brain imaging. They are not useful for extremity imaging such as the imaging of knee joints as the joint cannot reach to the imaging zone for most patients due to the long distance to the dsv from the end and the difficultly of placing and accommodating the other leg, for example.
Frusto-conical magnets for MRI are disclosed in U.S. Pat. Nos. 5,307,039 and 7,498,810. However, the magnet of U.S. Pat. No. 5,307,039 still has the disadvantages of being a relatively large magnet but only produces a field strength of around 0.5 T. While the magnet of U.S. Pat. No. 7,498,810 is smaller, and provides access from both ends, its configuration is particularly suited for extremity imaging rather than imaging of human heads.
The present invention aims to provide relatively small and therefore inexpensive magnets and magnetic resonance systems for imaging of human heads, extremities as well as neonates, etc.